Filtering arrangement



C- 8, 1945- A. M. GuREwlTscH FILTERING ARRANGEMENT Filed July so, 1942 Inventor; Y Anaiole M. Gurewitsch,

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Patented Oct. 8, 1946 2,408,927 FILTERING ARRANGEMENT Anatole M. Gurewitsch, Schenectady, N. Y., assgnor to General Electric Company, a corporation of New York Application July 30, i942, serial-No. 452,946

Claims.

This invention relates to an electrical filtering arrangement especially adapted for use at ultra high frequencies.

As is well known, it is frequently necessary in the design of radio circuits and like apparatus to provide means for effectively separating the direct current and alternating current paths. At the lower frequencies, such as the frequencies commonly used in radio broadcasting, this separation may be accomplished by the use of choke coils and condensers, properly arranged. However,V at higher frequencies, especially at frequencies corresponding to wave lengths on the order of a few centimeters, the conventional arrangements fail to produce the desired results and it is necessary to resort to more complex expedients including specially designed lters.

The present invention is principally concerned with the provision of an improved form of ultra high frequency lter which is characterized by simplicity and compactness of structure and which is well adapted for use at even the shortest wave lengths.

Briefly, the invention makes use of a multiplyfolded coaxial transmission line arrangement providing a plurality of quarter wave filter sections in series. In order to obtain a band-block characteristic the various sections may be made of different lengths so that each corresponds to a quarter wave length at some particular frequency within the selected frequency range.

The features of the invention desired to be protected herein are pointed out in the appended claims. The invention itself, together with its further objects and advantages, may best be understood by reference to the following descripy tion taken in connection with the drawing in which Fig. 1 is a longitudinal sectional view of a high frequency oscillator embodying one of my improved filter arrangements; Fig. 2 is avpartial section of the discharge tube which is represented in less detail in the assembly of Fig. 1, and Fig. 3 is a diagrammatic view useful in explaining the operation of the construction of Fig, l.

Referring particularly to Fig. 1 there is shown an ultra-high frequency oscillator having a three-electrode discharge tube l0 as its central element. As appears more clearly in Fig. 2, this tube comprises a cylindrical anode ll, a grid l2 and a cylindrical cathode I3 having its emissive part in the form of a flat disk surface I4 which faces the grid |12. The envelope within which these electrodes are enclosed comprises a series of three circular metal parts I6, l1 and I8 which are supported in mutually spaced relation by 2 glass cylinders 20 and 2l sealed between them. The part l1 provides a terminal for the grid l2, and a terminal for the anode is provided by a cylindrical enlargement 23 which is welded or soldered against the upper surface of the part I6.

The cathode I3 is provided at its lower extremity with a flange 25 which parallels the under surface of the part I8 but which is separated from that part by an insulating spacer 21 (e. g. a mica washer). With this arrangement the part I8 has a high frequency connection with the cathode through the capacity existing between this part and the flange 25 but is effectively insulated from the cathode as far as direct current is concerned. Separate direct-current connections are made to the cathode by means of leadin wires 29 secured to the under surface of the part 25 and terminally connected to prongs 3l which depend from the base of the discharge tube. Additional prongs 33 and lead-in wires 34 are provided for the purpose of supplying heating current to a coiled filament 35 arranged within the cathode cylinder I3 so that the emissive part of the filament can be maintained at an elevated temperature by this means.

In the arrangement of Fig. 1 the tube l0 is seated in a socket or adaptor 31 having accessible terminals 38 to which appropriate con-fr nections may be made. The socket 31 is itself supported by the combination of a xed ring 40 and a split locking ring 4l, the ring 40 being secured to the inner surface of an elongated cylinder sleeve 44 having certain further functions Which will be explained at a later point.

The oscillating circuits employed in connection with the discharge tube l0 are provided by the combination oi a series of three concentric tubular conductors 48, 49 and 58. The outer conductor 48 has a high frequency connection with the cathode of the tube l0 through a ring of contact fingers 53 which bear upon the surface of the tube part I8. Similarly, the cylindrical conductor 48 has a symmetrical connection with the grid of the tube through spring contact fingers 5.5 which bear upon the grid terminal l1. Finally, a high frequency connection between the inner tubular conductor 50 and the anode of the tube l0 is provided by pressing the extremity of the conductor over a tapered adaptor 51 which is fitted snugly onto the anode terminal and which bears contact lingers 51 adapted to engagethe disk I6. The tubular conductor 50 is insulated from the adaptor 51 with respect to direct currents by the interposition between their juxtaposed surfaces of a mica sleeve 58. It will be seen, however, that the capacity which exists through the mica assures that the conductive parts in question are effectively directly connected as far as high frequency currents are concerned.

With the arrangement specied, the space between the conductors 4B and 49 forms in effect a transmission line section which may be adjusted as to length to provide a resonant tank having a resonant frequency appropriately related to the desired frequency of operation of the oscillator. This adjustment may be accomplished by the use of an annular plug orcontact ring 6G having two sets of contact ngers El and 62 which respectively bear upon the opposed surfaces of the tubular conductors `lland Q9. Actuating rods 54 which extend out through a A similar tunable transmission line section iS provided between the grid and anode by the combination of the tubular conductors 49 and 50. The resonant frequency of the tank circuit thus provided betweenthe grid and anode is adjustable, as in the case of the grid-to-'cathode circuit, by means of a slidable contact ring or plunger l(53,' having Vexternally Assuming the application of appropriatedireot current potentials to the various electrodes of theturbe iiioscillations may be obtained by properly adjusting 4the two tunable tank circuits providedand by arranging for a properfeed back coupling between thesencircuits. The principles which govern the operation of an` oscillator of this kind arel fully described in rial No. 448,205, nieu of Richard C. Jensen assigned to the General Electric Company, a corporation of New York. In quired that the inner and outer lines betuned operating frequency, such tuning being obtained by adjusting each` line so that its length corresponds approximately to an odd number (e. g. l, 3, 5, etc.) of quarter wave lengths at the frequency to which it is being tuned. Feedback may in some instances be provided Asolelyby the inter-electrode capacity of the tube l i) butin most `cases it is found preferable to employ a probe o1* loop (not shown in Fig. l) extending betweenj the inner and outer transmission line sections for providing more positive coupling between them. jOn the assumption that the concentric conductors 58, 49, 5G and the connectedmetallic parts` provide a Vcomplete conductive enclosure, it would appear that high frequency energy developed within the enclosure `should be confined and that no high frequency coupling between the oscillating system and external agencies should exist. However, such an assumption ignores the presence of the insulating spacers 2l (Fig. 2) and 58 (Fig. l). These spacers, which are required to permit the establishment of D.C. potential differences between the various electrodes of the tube l, represent leakage gaps in an otherwise continuous metallic structure and may permit uncontrolled radiation of high frequency energy and other disadvantageous effects. For this reason it is necessary to provide filtering means in association with the D.-,C. and

low frequency A.-C. connectionsof the tube.Y

The grid i4 of the tube is connected directly accessible actuating rods 69.

adaptor 5l by the spacer Yency for coupling to exist between the otherwise to the main structure of the oscillator and is operated at ground potential, so that no diiculty exists with reference to the D.-C. connections to this element. On the other hand, mutually insulated connections must be made to both the anode l! and the cathode I3.

The anode terminal is insulated from the grounded structure by the mica spacer 58, and positive potential is applied to it by the use of a conductor l2 connecting with the adaptor 51 and extending outwardly through an insulating block '5... at the end of the oscillator structure. an externaily accessible terminal for this arrangement being indicated at l5. Due to the gap providedbetween the tubular conductor 50 and the 58 there is some tendcompietely enclosed high frequency system and the direct current conductor T2 and its associated circuits. In order to avoid objectionable effects attributable to this cause, it is expedient to provide filtering means in the formrof hollow metal seeves i8 and 78 tted on the conductor l2 and each providing a reentrant discontinuity with respect to currents ilowing along this conductor- Filtering means of the type specified are known `er se and are not intended to be claimed herein except as embodied in an improved arrangement to be described at a later point. Their usefulness depends in part upon'the consideration that the conductor "i2 and the surrounding tube 5i) form a coaxial transmission line to which the propagation of high frequency energy is necessarily conned. The passage of such energy along the line may be alternatively viewed as attributable to high frequency currents flowing over` the opposed surfaces of the coaxial conductors orto the propagation of waves in the space between the conductors. Taking the latter viewpoint, it may be'said that the discontinuities provided by the sleeves 'I8 and l@ tend to block the propagated waves by reflecting them For complete blocking or reflectionto 'occur at a particular frequency, the optimum condition is that in which the length of the blocking sleeve is a quartel` wave length (or an odd number of quarter wave lengths) at `such, frequency and in which the impedance seenl at the righthand end of the sleeve is infinite. The latter condition is fulfilled `by the reentrant'sleeve construction illustrated in Fig. 1 since the resonant quarter wave section enclosed by each sleeve actually represents an innite impedance (at the resonant frequency) in series withy the section formed between the outer surface of the sleeve andthe surrounding conductor. It is also desirable that the difference in diameter between each sleeve and the conductor 'l2 be relatively great and that the spacing between the sleeve and the surrounding conductor 5l be small. By making the sleeves 78 and 'I9 of different lengths and by using as many such sleeves as are needed, a considerable band of frequencies may be blocked so that unwanted coupling is prevented regardoscillator is operated.

A generally Vsimilarproblem with respectto high frequency coupling exists in connection with the means employed'for supplying heating current (either D.-C. or 6o cycle'A.-C.) tothe cathode of the tube iin These means include the socket terminal 38', which. is assumed to be connected to the cathode filament, and a conductor having a connection with this terminal (the other cathode connections do not appear in Fig.

|J,.through the opening Si' and along 1, but are indicated diagrammatically in Fig. 3). .The `conductor 88 extends one vend it is supported in an insulating bushingv 90,' and at vthe other end is provided with an externally accessible terminal 9| mounted in an insulating wall part92. Objectionable high frequency coupling is to be anticipated in connection with this arrangement becausev of the possible propagation of high frequency energy through the gap provided by the cathode spacer 21 (Fig. 2), through the glass seals of the tube the conductor 86.

VThis problem may, of course, be dealt with by a filtering arrangement identical with that employed inV connection with the anode, that is, by the use of several reentrant quarter wave iilters arranged sequentially in axial alignment. However, while the use of such an arrangement in connection with the anode does not tend materially to increase the space requirements of the oscillator, the same consideration does not apply in connection with the cathode; that is to say, the use of two or more quarter wave lters arranged in end-to-end relation in connection with the cathode terminal would obviously greatly extend the length of the structure as a whole.

In accordanceV with my invention this difficulty is avoided by the use of a folded conductor arrangement in which a number of quarter wave filter sections are provided in a concentric relationship. This arrangement includes in the first instance a conductive element 93 which surrounds the conductor 86 and which for a short portion of its length forms with the conductor a coaxial transmission line along which high frequency energy passing through the opening 8l tends to be propagated. To block the ow of such energy a reentrant quarter wave discontinuity is provided by mounting upon the conductor 86 a metallic sleeve 94 which is open at one end and which is of materially larger diameter than the conductor. The length of the sleeve 94 is made such as to correspond closely to one or more odd quarter wave lengths at a selected frequency Within the expected operating range of the oscillator. In series with this "arrangement there is provided a second reentrant quarter wave filtering section provided by a tubular conductor 91, which may be looked upon as a reversely bent extension of the conductor 93. This is surrounded by a tubular conductor 98 which bears a reversely bent part |08 adapted to provide a still further reentrant discontinuity in the path of wave propagation. A sleeve fitted into the cylinder 44 decreases the dimensions of the free space about the conductive part l. A metallic washer |03 which closely surrounds the extremity of the conductor 86 serves to support the associated tubular elements 98 and |80 and closes one end of the inner conductor. For supporting this structure and for securing it to the remaining structure of the oscillator, use is made of a clamping ring ||0 which abuts against a narrow flange I|| provided on the outer surface of the part d4 and which has a screw-threaded engagement with the part 43 as indicated at ||2.

As a result of the Aarrangement described, the only possible path for the escape of high frequency energy through the left hand extremity of the apparatus of Fig. 1 is by series passage through the various filter sections. Where it is ing to a quarter wave 4desired to operate the oscillator at one frequency ,only, all the filter sections may be made of identical length, that is to'say, of a length correspondlength at the frequency in question. On the other hand. where a range of operating frequencies is to be employed, the various iiltering sections are preferably made of different lengths as shown, so that the arrangement as a whole provides a band-stop filter.

The operation of the invention may be 4further explained by reference to the diagrammatic representation of Fig. 3 in which elements which can be readily identified with parts previously described in connection with Figs. 1 and 2 are designated by similar numerals differentiated by priming. l

yAs has been previously explained, the oscillating system is made up of the discharge tube I8 and the two resonant circuits which are respectively provided between thenconductorsrl'' and 49 and the conductors 49 and 50. Feedback is provided (in part) by means of a conductive probe ||8 which is mounted on the tuning ring 6G' and which extends through an axial slot cut in the tubular conductor A9'. An output arrangement for the high frequency energy developed is illustrated as comprising a coupling loop H6 which extends into the space between the conductors 49 and 50 and which is made externally accessible by means of a concentric conductor transmission line comprising a metal tube H8 and a coaxial wire |20.

Direct current terminal connections for the electrodes of the tube I8 are indicated at 12 and 86', cathode bias being provided by a resistor |5 connected in series with the cathode. The considerations which require the use of filtering arrangements in association with these connections arise from the presence of the condensers |2| and |22, the first set representing the by-pass capacity provided by the insulating washer 21 (Fig. 2) and the second set representing the capacity provided by the mica insert 58 (Fig. 1). As has been previously indicated, these spacers represent gaps through which undesirable high frequency coupling may occur. As far as the anode conductor 12' is concerned such coupling is prevented by the inclusion of the filtering elements 18 and 19. In connection with the cathode supply conductor 86 a similar function is performed by the concentrically arranged lters 96 to 10|. Accordingly, no substantial coupling between the high frequency oscillator and the external circuits is permitted.

While the invention has been described by reference to a particular embodiment it will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come Within the true spirit and scope of the foregoing disclosure. In the claims, wherever the term quarter wave is employed it is to be understood as applying either to a single quarter wave or to any odd number of quarter waves.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A filter comprising a plurality of successively telescoped coaxial conductors connected to provide a plurality of quarter wave discontinuities in series,` adjacent ones of said conductors being insulated from each other for unidirectional currents.

2. A band-stop filter for frequencies within a 7 given range comprising a plurality of successively telescoped concentric conductors *connected to provide a number of filtering sections in lseries,

'adjacent ones of said conductors being insulated from each other for unidirectional currentsy each of said sections having a length corresponding to a quarter wave length at some frequency within the given range, and the various sections being of diierent diameters whereby each represents a discontinuity with respect'to the others.

3. In anarrangement for decoupling the power leads to an ultra high frequency space resonant system, a high frequency lter including a plurality of telescoped conductors of varying diameters connected to provide a number of filtering sections in series, each of said sections comprising a reentrant quarter wave discontinuity in the l path of propagation of the waves desired to be filtered, and adjacent ones of said conductors being insulated from each other for unidirectional currents.

4. A device for decoupling the power leads to an ultra high frequency space resonant system comprising, a high frequency lter including a plurality of telescoped conductors arranged to provide a multiple reversed path forelectromagnetic waves tending to pass through the lter, each reversed section of the filter comprising a reentrant discontinuity effective to block passage of waves of a particular wave length, and adjacent ones of said conductors being insulated from each other for unidirectional currents.

5. In combination, a high frequency system, a conductor for supplying current to a. pait of said system, and a high frequency filter associated with said conductor, said lter comprising a conductive tube surrounding said conductor and defining with it a'coaxial transmission line, means on said conductor for providing a quarter wave discontinuity in saidrline, and further conductive means concentrically surrounding said tube and insulated therefrom at the frequency of `said current for providing a, second quarter wave discontinuity serially related to the first.

ANATOLE M. GUREWITS CH. 

